The present invention is an acoustic capacitor that is intended for use in a "pressure jump" detector. A "pressure jump" is the sudden rise in atmospheric pressure that accompanies the outflow from a thunderstorm; these pressure jumps have been found to indicate the wind shears accompanying such storms, hence measurement of them is useful around airports which are subject to such storms. (For additional material on pressure jumps in general see "The Design and Use of Sensitive Pressure-Jump Sensors to Detect Thunderstorm Gust Fronts. Part 1: Pressure-Jump Detector Design" by Bedard and Meade, Journal of Applied Meteorology, Vol. 16, No. 10, October 1977.) Pressure jumps are of the order of 0.5 to 2 millibars and occur over time spans of 2 to 5 minutes. Measuring them must take into account the instantaneous atmospheric pressure, since a pressure jump can start from any level of pressure. Therefore the reference pressure against which the pressure jump is measured must be the atmospheric pressure which existed just prior to the jump, rather than a vacuum or other fixed pressure.
Early pressure jump detectors used a 50-100 gallon container for the reference pressure; sometimes this was buried underground for thermal stability. Obviously a large container such as that was difficult to conveniently install in most locations, hence a smaller reference pressure container was designed. This container had a volume of about 0.25 liters and was filled with a porous thermally conductive material such as steel wool. The porous thermally conductive material helped somewhat to suppress the short-period thermally induced pressure changes because of its increased heat capacity.
While the steel wool-filled reference volume worked reasonably well, it suffered from problems with variation in the pre-set trigger point. That is, the minimum level of pressure jump which would activate the detector would vary with atmospheric (and hence detector) temperature. Thus it was necessary to come up with a design which was small and easily mounted, but which did not have the objectionable temperature dependence of the prior models.
All reference pressure containers have a small capillary leak which allows the internal pressure to follow the long-term barometric pressure fluctuations; increasing the size of this capillary would reduce the thermally-induced pressure fluctuations, but it would also reduce the sensitivity of the detector to pressure jumps.